Dewaxing processes of various kinds are widely used in the pertroleum refining industry to improve the fluidity at low temperatures of various petroleum fractions, including fuels such as jet fuel, kerosene, home heating oil, diesel fuel as well as lubricants, where the necessity of procuring good low temperature performance is particularly pressing because the paraffinic nature of lubricants coupled with the high molecular weight of the lubricant fractions generally implies a relatively high wax content which will lead to poor low temperature performance unless adequate remedial measures are taken. Originally, dewaxing was carried out by physical techniques, especially solvent dewaxing using solvent mixtures such as MEK/toluene or by autorefrigerant processes such a propane dewaxing. More recently, however, catalytic dewaxing processes have established themselves commercially. Catalytic dewaxing processes are available both for fuels and for lubricants. The Mobile Distillate Dewaxing process (MDDW) is useful with fuels such as jet fuel and diesel fuel and the Mobile Lube Dewaxing process (MLDW) with lubricants, including distillate (neutral) and residual (bright stock) types. The MDDW and MLDW processes both employ shape-selective cracking to remove waxy components, mainly normal and slightly branched chain paraffins, from the feed to produce a dewaxed product having a reduced pour point which is dependent on the severity of the processing. The MLDW process employs a second reactor containing a hydrofinishing catalyst which ensures that the dewaxed lube product meets all applicable quality and engine performance criteria, as described in "Industrial Application of Shape-Selective Catalysis": Catal. Rev.-Sci. Eng. 28 (2-3) 185-264 (1986), especially page 244. See also Refining Process Handbook (Hydrocarbon Processings, September, 1986), pages 89,90 and, as an example of an early proposal in this field, Oil and Gas Journal, 6 January 1975, pages 69-73.
Catalytic dewaxing processes selectively remove the longer chain, waxy paraffins, mainly n-paraffins and mono-methyl paraffins from the feed. Most processes of this type operate by selectively cracking the waxy paraffins to produce lower molecular weight products which may then be removed by distillation from the higher boiling lube stock. The catalysts which have been proposed for this purpose have usually been zeolites which have a pore size which admits the straight chain, waxy n-paraffins either alone or with only slightly branched chain paraffins but which exclude the less waxy, more highly branched materials and cycloaliphatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35 and ZSM-38 have been proposed for this purpose in dewaxing processes, as described in U.S. Pat. Nos. Re 28,398 (3,700,585); 3,582,189; 3,894,938; 4,176,050; 4,181,598; 4,222,855; 4,229,282; 4,287,388; 4,259,170; 4,283,271; 4,283,272; 4,357,232 and 4,428,819 to which reference is made for details of such processes. A dewaxing process employing synthetic offretite is described in U.S. Pat No. 4,259,174. A process using a mixture of zeolites of different pore sizes is disclosed in U.S. Pat. No. 4,601,993. Reference is made to those patents for details of such processes.
The catalytic dewaxing processes using intermediate pore size zeolites such as ZSM-5 operate, as described above, by selectively cracking the waxy components of the feed. This results in a loss in yield since the components which are in the lube boiling range undergo a bulk conversion to lower boiling fractions which, although they may be useful in other products, must be removed from the lube stock. Another approach to processing of lube stocks is described in U.S. Pat. Nos. 4,419,220 and 4,518,485, in which the waxy components of the feed, comprising straight chain and slightly branched chain paraffins, are removed by isomerization over a catalyst based on zeolite beta. During the isomerization, the waxy components are converted to relatively less waxy isoparaffins and at the same time, the slightly branched chain paraffins undergo isomerization to more highly branched aliphatics. A measure of paraffin cracking does take place during the operation so that not only is the pour point reduced by reason of the isomerization but, in addition, the heavy ends undergo some cracking or hydrocracking to form liquid range materials which contribute to a low viscosity product. The degree of cracking is, however, limited so as to maintain as much of the feedstock as possible in the desired boiling range.
The catalysts used in these processes have invariably included a binder in addition to the zeolite, at least when the process was to be operated on a commercial scale. There is a number of reasons for this. First, in a commercial scale unit, a considerable depth of catalyst is maintained in the conventional trickle-bed, downflow reactor so that the catalyst at the bottom of the bed requires significant crushing resistance to withstand the weight of catalyst above. It also requires considerable attrition resistance to withstand the stresses of handling during manufacture and loading into the unit. A further consideration is that is should be practicable to produce the catalyst in the desired particle size and shape using conventional equipment and this requirement generally implies that the catalyst should be capable of being produced by extrusion.
Extrusion is one way of obtaining a material which has a high degree of strength for various applications, both catalytic and noncatalytic. Some aluminosilicates zeolites have long been used as catalysts for a wide variety of organic conversion processes and, in general, crystalline aluminosilicate zeolites are incorporated with a matrix or binder material in order to impart strength to them. The most commonly used matrix materials have included alumina and mixtures of alumina with clays because these materials were very easy to extrude and resulted in the production of an extrudate which had desirable physical strength.
Silica is known to be a desirable matrix and posesses advantages over alumina in certain catalytic reaction. For example, U.S. Pat. No. 4,013,732 discloses ZSM-5 with a silica matrix, see Column 7. U.S. Pat. No. 3,843,741 and U.S. Pat. No. 3,702,886 would broadly teach the use of ZSM-5 with a silica matrix. Thus, although it was considered desirable to composite ZSM-5 with a silica matrix, it was not possible to do this by an extrusion because silica and zeolites will not extrude in conventional extruding equipment to give reasonably strong products. The only way that composites of ZSM-5 and silica could be made was by a pilling or pelleting which involved mixing silica and the appropriate zeolite and squeezing it together to form a shaped structure having minimum physical strength.
Catalysts may be produced from silica gels. U.S. Pat. No. 3,969,274 describes the advantages of having silica as a support for catalysts but utilizes a silica gel which has been subjected to steam treating in order to enhance its crush strength. A steaming step is stated to be required in order to make the process operable.
Silica-zeolite pellets or extrudates may be treated with various materials in order to increase their strength. For example, U.S. Pat. No. 3,846,337 discloses silica-bound silicate particles of improved crush strength which can be prepared by admixing reactive silica sols with siliceous particles, and contacting the resulting combination with ammonium phosphate, an acid phosphate or both.
U.S. Pat. No. 4,111,843 describes the preparation of porous silica particles by adding excess alkali to a water glass and then precipitating a hydrogel with acid. The acess alkali is stated to be responsible for increasing pore diamter. The patent discloses (Column 2, line 15), that the extrudability of microporous silica-alumina catalyst carriers of exceptional strength can be achieved by following the process of the patent. However, the patent does not disclose anything with regard to extrudates of silica particles, (see Column 6, line 66 and following).
U.S. Pat. No. 4,582,815 and its corresponding European Publication No. 167324 disclose a method for making extrudates of exceptional strength, by mulling either pure silica, or a crystalline aluminosilicate zeolite, or more preferably, a mixture of silica with a crystalline aluminosilicate zeolite with water to a solids level of 25 to 75 percent in the presence of 0.25 to 10 weight percent of a basic material such as sodium hydroxide (calculated as solid sodium hydroxide and based on the total solids present).
It is theorized in U.S. Pat. No. 4,582,815 that the successful extrusion or formation of shaped bodies by wetting and compressing powders requires that the particles be capable of being brought into close proximity with each other so that the van der Waals forces become operative in subsequently holding the particle together. Chemical binding by crosslinking may also occur for some binders but the initial requirement is still for close packing. Highly siliceous materials like silica and zeolites of high silica-to-alumina ratios are hydrophobic. It has been discovered that by substituting an alkali metal for hydrogen in the silanol groups on the outside surfaces of siliceous materials they can generally thereby be made more easily extrudable with crush strengths far exceeding those heretofore described for silica-bound materials.